The present invention relates to a blood pump system for pumping blood as a continuous flow.
In recent years, development of continuous flow type blood pumps as artificial heart units has been conducted energetically. A general example of continuous flow type blood pump is the one of the type in which an impeller is rotated by a motor to thereby pump blood. Another example is a so-called axial flow pump in which a shaft is rotated by a motor to thereby pump blood. For measuring the blood flow rate in these pumps, there may be considered the following two methods:
(1) To attach a flow rate sensor.
(2) To calculate the flow rate from information on the motor.
The method of (1) is advantageous in that accurate flow rate measurement can be achieved, while the method of (2) is advantageous in that it is needless to provide a flow rate sensor and, hence, it is possible to reduce the apparatus in size and to reduce power consumption. Therefore, an apparatus of the type of (2) is more desirable.
In the method of (2), the flow rate of the pump is calculated, for example, from the rotational speed of the motor, the motor current, or the like. Taking a motor speed constant control (generally, a motor rotational frequency fixing control is conducted) as an example, the relationship between the motor current value and the pump flow rate at a flow rate of not less than 0 is a monotonous increase relationship, which makes it possible to calculate the pump flow rate from the motor current value. In this case, the pump system stores a flow rate operation expression, and the flow rate is calculated from an actually measured motor rotational speed and an actually measured motor current or the like by use of the operation expression.
As an system of the type (2) above, a centrifugal fluid pump system is disclosed in U.S. Pat. No. 6,142,752.
In the publication, there is disclosed the centrifugal fluid pump system in which a controller stores either flow rate-related data preliminarily obtained through measurement of the relationships between the motor current of the fluid pump system, the motor rotational speed and the flow rate or relational expression data calculated from the related data, and has a flow rate calculation function for calculating the flow rate by use of the motor current value, the motor rotational speed, the relational expression data, and the fluid viscosity calculated by a fluid viscosity calculating function.
In the case of pumping blood by a pump, it is necessary to prevent a backflow of the blood. Where a blood pump system is used as a left ventricular assist device, a backflow of blood is generated, for example, for the following reason. There is a period in which the aortic pressure is higher than the left ventricle pressure on the side of a living organism. For example, in the case where rotary pump assists pumping function from left ventricle to aorta, a backflow occurs if the pump head in this period is low. Such a backflow in the blood pump corresponds to the condition where the pump is out of function in the period.
In the system shown in the above-mentioned publication, a favorable calculation of the flow rate (flow rate) is possible under normal conditions. However, in a negative-flow-rate zone (the conditions for backflow), it is impossible to arithmetically calculate, or detect, the backflow by use of the flow rate operation expression which is prepared for a positive-flow-rate zone.